1. Field of the Invention
The present invention relates to a sensor deterioration judging apparatus and a sensor deterioration judging method for judging whether a gas sensor is in a deteriorated state.
2. Description of the Related Art
A gas sensor is known which is equipped with a first measurement chamber, a first oxygen ion pump cell, a second measurement chamber, a second oxygen ion pump cell, a reference oxygen chamber, and an oxygen partial pressure detection cell.
Various apparatus for judging the state of such a gas sensor have been proposed which measure currents flowing through various cells of the gas sensor, output voltages of cells, impedances of cells, or like parameters and detects a failure state of the gas sensor on the basis of whether or not measurement results are within allowable ranges (refer to JP-A-11-014589 (corresponding to U.S. Pat. No. 6,120,663) and WO-A-03/083465, for example).
3. Problems to be Solved by the Invention
However, the above conventional apparatus have a problem that they cannot detect a deteriorated state of a gas sensor such as low sensitivity of a cell though they can detect fatal failure states such as a line disconnection and short-circuiting among various states of a gas sensor.
In a gas sensor having a fatal failure, the values of currents, voltages, impedances, etc. of various cells are clearly out of the ranges of those of a normal gas sensor. Therefore, a failure state of a gas sensor can be detected by the above conventional apparatus.
In contrast, in a deteriorated gas sensor, the values of currents, voltages, impedances, etc. of various cells are approximately in the same ranges as those of a normal gas sensor. Therefore, it is difficult to discriminate a deteriorated state from a normal state on the basis of the above values. Therefore, it is difficult for the above conventional apparatus to detect a deteriorated state of a gas sensor.
Having a different a gas detection characteristic than in a normal state, a gas sensor in a deteriorated state cannot produce the same gas detection result as in the normal state and hence may be lowered in gas detection accuracy.
The present invention has been made in view of the above problem, and an object of the invention is to provide a sensor deterioration judging apparatus and a sensor deterioration judging method capable of judging whether a gas sensor having various cells is in a deteriorated state.